swift 3.0 bug or deliberate behavior? At minimum a confusing error code.

Contents.swift

//: Playground - noun: a place where people can play

import UIKit

var str = "Hello, playground"

protocol namer {
    func name() -> String
}

extension namer {
    func name() -> String {
        return "foo"
    }
}

class DemoClass: namer {
    
    init () {
    
    }
    
    func demoNameInstance() -> String {
        let n = name()
        return n
    }
    
    class func demoNameClass() -> String {
        let n = name()
        return n
    }

    
}

let a = DemoClass()
let b = a.demoNameInstance()
let c = DemoClass.demoNameClass()